1. Field of the Invention
This Invention relates to a bandpass filter having a plurality of resonators formed of strip lines.
2. Description of the Prior Art
A bandpass filter having resonators used in microwave or millimeter wave integrated circuit is known. In such bandpass filter, open-end strip lines are used for such resonator because high frequency grounding is unnecessary and variation in resonating frequency is low. Such resonator employs a uniform strip line because of easiness of design. Coupling at input/output and between those resonators are obtained by electromagnetic coupling at side edges of strip lines and electrostatic coupling at end edges of strip lines.
FIG. 4 is a plan view of such first prior art bandpass filter. Numerals 1, 2, and 3 denote uniform strip line type cascaded resonators. Numerals 4 and 5 denote input/output strip lines for coupling the resonator 1 or 3 to external circuits by electromagnetic coupling.
thereinbelow will be described operation of this prior art bandpass filter.
A high frequency signal inputted from the input/output strip line 4 is coupled to the resonator 1 by distributed coupling through side edges of the input/output strip line 4 and the resonator 1. The resonators 1, 2, and 3 are coupled through side edges and outputs a filtered high frequency signal at the input/output strip line 5 through coupling between the resonator 3 and the input/output strip line 5. Thus, this prior art bandpass filter provides a frequency characteristic determined by the resonance frequencies of the resonators 1, 2, and 8, etc.
FIG. 5 is a plan view of a second prior art bandpass filter using electrostatic coupling between end edges of strip lines. Numerals 6, 7, and 8 are uniform uniform strip line type cascaded resonators. Numerals 9 and 10 are input/output strip lines using electrostatic coupling between ends of strip lines.
Hereinbelow will be described operation of this prior art bandpass filter.
A high frequency signal inputted from the input/output strip line 9 is coupled to the resonator 6 by distributed coupling through end edges of the input/output strip line 9 and the resonator 6. The resonators 6, 7, and 8 are coupled through end edges of strip lines and outputs a filtered high frequency signal at the input/output strip line 10 through coupling between the resonator 8 and the input/output strip line 10. Thus, this prior art bandpass filter provides a frequency characteristic determined by the resonance frequencies of the resonators 6, 7, and 8, and coupling characteristics, etc.
In the above mentioned bandpass filter using magnetic coupling between side edges of strip lines, there is a problem it is impossible to arrange the resonators 1, 2, and 3 and the input/output strip lines 4 and 5 in line. That is, this prior art band pass filter extends two-dimensionally.
In the above mentioned bandpass filter using electrostatic coupling between end edges of strip lines, there is another problem that a total length of the bandpass filter is long because the resonators 6, 7, and 8 and the input/output strip lines 9 and 10 are arranged in line. Moreover, there is further problem that it is difficult to obtain a broadband characteristic in the bandpass filter using electrostatic coupling between end edges of strip lines because electrostatic coupling between end edges of strip lines cannot provide sufficient degree of coupling.
Moreover, when these bandpass filters are used as an output filter in a non-linear circuit such as an oscillator, amplifier, or the like, there are provided other passbands at frequencies natural number times desired frequency because uniform strip line is used in the resonators. Therefore, there is a problem that it is necessary to use these bandpass filter in combination with other bandpass filter having different frequency characteristic.
Hereinbelow will be described a third prior art bandpass filter.
An interdigital bandpass filter of high frequency band, comprising quarter wave strip line resonators is known.
FIG. 9 is a perspective view of such a third prior art interdigital bandpass filter. Numerals 101 and 102 are input/output terminals. Numerals 103 to 105 are resonators formed on a dielectric substrate 106. Numerals 107 and 108 are input/output coupling lines. Numeral 109 is a ground electrode. These lines comprise strip lines or microstrip lines.
Hereinbelow will be described operation of the bandpass filter mentioned above.
A high frequency signal inputted to the input/output terminal 101 is coupled to the resonator 103 by distributed coupling. The resonators 103, 104, and 105 formed on a dielectric substrate 106 are coupled through end edges of strip lines and outputs a filtered high frequency signal at the input/output strip line 108 through coupling between the resonator 105 and the input/output strip line 108. Thus, this prior art bandpass filter provides a frequency characteristic determined by the resonance frequencies of the resonators 103. 104, and 105 and coupling characteristics, etc.
However, in the structure mentioned above, that is, in the interdigital bandpass filter, if its passband is required to be narrow, there is a problem that gaps between the resonators should be made wider, so that its size becomes large. Moreover, there are also problem in assembling and trimming because open-ends of the resonators cannot be subject to trimming because the open-ends of the resonators arranged not in the same direction.
Moreover, in the comb-line structure, there is also a problem that either coupling of electromagnetic or electrostatic couplings should be stronger than the other by providing partitions between the resonator, etc. because in the comb-line structure electromagnetic and electrostatic components have an antiphase relation.